The invention relates to a lung inflection point monitor and method, and more particularly to a monitoring device and method for finding the critical opening pressure, the critical closing pressure and the over distention pressure of the lungs.
When a patient is ill and requires help breathing, a mechanical ventilator is used. The ventilator typically assists in the breathing process by first delivering a volume of air to the lungs during the inspiratory phase of the breath, and then allowing gas to passively evacuate during the expiratory phase of the breath. During this process gas is exchanged in small air sacks called alveoli. It is in the alveoli that blood becomes oxygenated and expels carbon dioxide, which is, in turn, removed during exhalation.
If a patient is sick enough to require mechanical assistance to breathe, often times a number of alveoli will collapse preventing gas from being exchanged. A goal of the clinician is to re-open these alveoli and keep them open during the ventilation process. The pressure point at which these alveoli open during the inspiratory phase of the breath is called xe2x80x9ccritical opening pressure.xe2x80x9d If the critical opening pressure is known, the clinician can program the ventilator so that there is sufficient pressure in the lungs at the end of the expiratory phase of the breath to keep the alveoli from collapsing. This pressure is called xe2x80x9cpositive end expiratory pressurexe2x80x9d or xe2x80x9cPEEP.xe2x80x9d
It is important to know the pressure point at which the lungs again collapse during exhalation. This pressure is referred to as xe2x80x9ccritical closing pressure.xe2x80x9d If the critical closing pressure can be ascertained, this pressure point can be correlated to the critical opening pressure allowing the clinician to fine tune the PEEP setting.
As gas is introduced into the lungs during the inspiratory phase of the breath, the lungs continue to expand. If more gas is delivered than the lungs can comfortably accommodate the lungs are stressed and over distend. This xe2x80x9cover distentionxe2x80x9d damages the lungs beginning on a cellular level and may escalate to the point of ripping holes in the lungs. The pressure at which the lungs begin to overfill is referred to as the xe2x80x9cover distention pressure.xe2x80x9d If the over distention pressure is known, the ventilator may be programmed to limit the amount of gas given during the inspiratory phase of the breath by setting a xe2x80x9cpeak inspiratory pressurexe2x80x9d or xe2x80x9cPIPxe2x80x9d at just below the over distention pressure.
The critical opening pressure, the critical closing pressure and the over distention pressure points are known as xe2x80x9cinflections points.xe2x80x9d By knowing the inflections points, the clinician can program the ventilator to keep the pressures during inspiration and expiration at levels that keep the airways open and prevent over distention. This reduces the risk of injury to the lungs and allows the ventilator to more efficiently ventilate the patient.
The importance of preventing both the collapse and over distention of the lung is well documented. In an article entitled xe2x80x9cOpen up the lung and keep the lung openxe2x80x9d by B. Lachmann, Intensive Care Medicine (1992) 18:319-321, a rationale for preventing airway collapse during ventilation is set forth in order to avoid the dangers concomitant with the pressures required to re-open the airways. The pressure necessary to open collapsed or partially collapsed airways creates dangerous shear forces which can deplete the alveoli of natural surfactant, damage capillaries, decrease compliance, and render gas exchange dysfunctional.
An article entitled xe2x80x9cInternational Consensus Conferences in Intensive Care Medicine: Ventilator-associated Lung Injury in ARDSxe2x80x9d, which represents a consensus report sponsored by The American Thoracic Society, the European Society of Intensive Care Medicine, the Societe de Reanimation de Langue Francaise utilizing consensus methods established by the National Institutes of Health, asserts that over inflation of the lung induces severe alveolar damage such as alveolar hemorrhage and hyaline membrane disease. The article suggests that ventilator modes associated with properly set PEEP, and delivered tidal volumes which result in a PIP below the over distention point, achieve significant reductions in mortality.
In U.S. Pat. No. 5,937,854, a ventilator pressure optimization method and device is described which attempts to optimize mechanical ventilation by finding the lung inflection points and transmitting this information to the ventilator. The method used involves delivering a known pressure to the patient and measuring the approximate resulting lung volumes. The volumes are then correlated to the delivered pressures and the inflection points are extrapolated. Although this method may provide information helpful in approximating the inflection points, the procedure necessitates measuring lung volume, a costly and difficult maneuver at best. Subsequently, the xe2x80x9capproximatexe2x80x9d lung volume is used to calculate the xe2x80x9capproximatexe2x80x9d volume differences as the pressure increases and decreases. These approximate volume differences are used to calculate the inflection points.
U.S. Pat. No. 5,575,283 describes a device for determining the opening pressure of the lungs by trying to measure the delivered lung volumes and trying to establish a relationship between those volumes and delivered pressures. Although different methods are used for trying to establish the lung volumes, the clinician still faces the same difficulties as enumerated in the methodology mentioned above.
U.S. Pat. No. 5,738,090 describes a system for determining the opening pressure of the lung by using a blood gas analyzer to measure partial pressures of oxygen in the blood. When the partial pressure reaches a xe2x80x9cpredetermined thresholds,xe2x80x9d the threshold is correlated to a pressure that is designated as the opening pressure. However, the partial pressures may or may not be indicative of open airways. The airways may be open, yet still not able to exchange gases due to an underlying pathology of the pulmonary, cardiac, or circulatory system. In addition, the partial pressures may be considered optimal at the very time irreparable, long-term damage is being done to the lungs.
The present invention is a method and apparatus for determining lung inflection points in order to optimally ventilate a patient on a mechanical ventilator. The method for determining lung inflection points includes the basic steps of: immobilizing the lungs; closing an exhalation valve; introducing gas into the lungs in pulsatile increments; measuring the resulting pressures; and determining the inflection points by analyzing a pressure-time line graph while observing overall pressure changes and incremental pressure changes.
The apparatus consists of a gas-controlling device, a device for measuring and displaying pressures and a monitor-patient interface. A microprocessor within the device is used for purposes of measuring, analyzing, displaying and transmitting data resultant from the maneuvers described herein.